Preliminary results from a nine-year study of water toxicity confirm the prevalent notion that discharges from agricultural and urban areas are fouling California's waterways in roughly equal measure, according to one of the report's authors.

"This report is saying that both agricultural and urban sources have a role to play in degrading water quality," said Dan Markiewicz, a University of California-Davis researcher and one of three co-authors.

Markiewicz said a follow-up report, to be published in early 2011, will contain direct comparisons of how much toxicity is coming from urban versus agricultural sources.

The study, conducted by UC-Davis researchers for the state's Surface Water Ambient Monitoring Program, found that most toxicity in the state's surface waters originates from organophosphate- and pyrethroid-based pesticides, both of which are used in landscaping and household products as well as farm applications.

The resulting toxicity occurs more uniformly across urban areas than agricultural areas, the researchers found. But a greater share of agricultural test sites -- about 35 percent -- were deemed "highly toxic" than were urban sites, at about 28 percent.

The report measures toxicity by whether fish, algae and other aquatic species can survive in water samples.

Researchers found declines in aquatic invertebrates downstream from where agricultural runoff enters waterways, including sites on the Salinas and Santa Maria rivers. They found similar effects in the Sacramento-San Joaquin Delta and the Central Valley waterways that flow into it, often in regions impacted by suburban development.

Researchers tested 992 sites, about half of which showed toxicity high enough to kill at least one test animal. Of the total, 13 percent were considered highly toxic, meaning the most sensitive test animals didn't survive.

The most rigorously tested region was the Central Valley, where 298 samples were taken.

"There is still some work to be done," Markiewicz said. "In the last 100 years, we've come a long way in improving water quality, but there is still a lot of toxicity."

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The report authors said they welcome input on which aspects of water toxicity should be addressed in their final report. To weigh in, contact: